Traditionally, it has been known in the field of rubber production to use a wet masterbatch in order to improve workability and filler dispersibility when producing a rubber composition containing filler such as carbon black and the like. The wet masterbatch refers to a material obtained by: mixing a filler-containing slurry solution and a rubber latex solution in a liquid phase, the filler-containing slurry solution being prepared by mixing beforehand the filler and a dispersion solvent in a specific ratio and dispersing the filler in the dispersion solvent by a mechanical force; and thereafter coagulating the mixture by adding thereto a coagulating agent such as an acid and the like, and recovering and drying the coagulated material.
When a wet masterbatch is used, there is provided a characteristic that, compared to a conventional rubber composition obtained by mixing in a solid phase, a rubber composition is obtained which has excellent filler dispersibility, and excellent rubber physical properties such as processability, a reinforcing property, and the like. By using as a raw material a rubber composition using a wet masterbatch, it can be expected, for example, that rubber products such as tire and the like, having reduced rolling resistance and excellent fatigue resistance and abrasion resistance, are produced.
According to a conventional method for producing a wet masterbatch, there have been some cases where an excessive shear force and impact force are exerted on the filler in a step of micronizing the filler slurry. When excessive force is exerted on the filler in the micronization step, there has been a problem that a structure of the filler is destroyed and its rubber reinforcing property deteriorates significantly. Therefore, in order to prevent destruction of the filler structure, micronization of the filler slurry may be suppressed. In this case, however, there arises a problem that dispersibility of the filler in the rubber composition becomes insufficient, and the rubber physical properties deteriorate.
In a field of an aqueous pigment ink, a technique of applying surface-treated carbon black has been proposed as a method for fining a particle size of a filler slurry such as carbon black and the like. However, this technique does not take into account the rubber reinforcing property and the like.
Furthermore, a wet masterbatch using a filler slurry micronized by surface treated carbon black has also been proposed. However, it has not been mentioned that there is an optimum region in carbon colloidal with regard to a relationship between surface functional groups and slurry micronization (see, for example, Patent Literature 1).